I already have a fully evolved Zeus and Poseidon...both as fighters and in my PvP. I also have an unevolved Zeus and Poseidon. Is there any benefit to evolving them (and subsequently having 2 of the same fighter), or should I get rid of them?

Hello! There can be benefits to having two of the same Fighter on your team, but not all Fighters share this quality.

For example, the Demon Fighters (Beezebub, Baphomet, Lucifer, Samael, Baal) give bonuses to the team depending on what Fighters you use on that team. They will grant you bonuses when using multiple Demons on your team or when using multiple Sin Fighters (Lust, Greed, Wrath, Sloth, Gluttony, Envy, Pride) on your team. So collecting & carrying multiples of either Demons or Sins is a pretty good idea. You could have a Yellow team of two Baphomet Captains, a Taranis, and three Gluttony, as an example.

Fighters like Zeus & Poseidon, on the other hand... There is not really a benefit to having multiples of these Fighters, other than to beef up your team's stats. Their Captain & Active Skills will not directly add anything to your team by having more than one of them. You could evolve them & use them add materials to level up their PvP forms if they are not already maxed out, but other than that, you do not necessarily need multiple copies of them.

The best Fighters to keep and use as multiples on a team are those that have Attack Boosting skills, like the Island Lords (Steam Buster, Agripa, Kraken, Erigs, Yeti), Leonidas, Legendary Ogre, Xi Wang Mu, Chimera, or even the Demons. If you have enough Energy, you can use two of these Fighters' Active Skills in one turn to really boost your Attack power!

You can also make use of duplicate Healers or damage reducers if you feel the need.

In the end, what Fighters you use on any given team depends on the goal you are trying to achieve. Use your own judgement & build a team you think fit to take on the varying challenges within the game.